run-tests.yml: implement docker layer caching

This GitHub Action caches all layers that are created after the step
where it is specified (which is why we pre-fetch some pull-only
images that we know we'll be using to run the tests).

The goal is faster CI by speeding up the build stage.

.github/workflows/run-tests.yml

@@ -15,6 +15,19 @@ jobs:
           # Needed by Codecov
           fetch-depth: 2
 
+      # Pull the latest image to build, and avoid caching pull-only images.
+      # (docker pull is faster than caching in most cases.)
+      - name: Download pull-only images
+        run: docker-compose pull mysql-integration-test flyway-integration-test
+
+      # In this step, this action saves a list of existing images,
+      # the cache is created without them in the post run.
+      # It also restores the cache if it exists.
+      - name: Docker layer caching
+        uses: satackey/[email protected]
+        # Ignore the failure of a step and avoid terminating the job.
+        continue-on-error: true
+
       - name: Run tests
         run: |
           composer run start:integration-services